Cannabis and mental health

Regular cannabis use increases your risk of developing a psychotic illness, such as schizophrenia. A psychotic illness is one where you have hallucinations (seeing things that aren't really there) and delusions (believing things that aren't really true).

Your risk of developing a psychotic illness is higher if:

you start using cannabis at a young age

you smoke stronger types, such as skunk

you smoke it regularly

you use it for a long time

you smoke cannabis and also have other risk factors for schizophrenia, such as a family history of the illness

Cannabis also increases the risk of a relapse in people who already have schizophrenia, and it can make psychotic symptoms worse.

Other risks of cannabis

Cannabis can be harmful to your lungs

People who smoke cannabis regularly are more likely to have bronchitis (where the lining of your lungs gets irritated and inflamed).

You're more likely to be injured in a road traffic accident

If you drive while under the influence of cannabis, you're more likely to be involved in an accident. This is one reason why drug driving, like drink driving, is illegal.

Cannabis may affect your fertility

Research in animals suggests that cannabis can interfere with sperm production in males and ovulation in females.

If you're pregnant, cannabis may harm your unborn baby

Research suggests that using cannabis regularly during pregnancy could affect your baby's brain development.

Regularly smoking cannabis with tobacco increases the risk of your baby being born small or premature.

Cannabis increases your risk of cardiovascular disease and stroke

If you smoke it regularly for a long time, cannabis raises your chances of developing these conditions.

Research suggests it's the cannabis smoke that increases the risk, not the active ingredients in the plant itself.

Does my age affect my risks?

Your risk of harm from cannabis, including the risk of schizophrenia, is higher if you start using it regularly in your teens.

One reason for this is that, during the teenage years, your brain is still growing and forming its connections, and cannabis interferes with this process.

Does cannabis have medicinal benefits?

Cannabis contains active ingredients called cannabinoids. Two of these – tetrahydrocannabinol (THC) and cannabidiol (CBD) – are the active ingredients of a prescription drug called Sativex. This is used to relieve the pain of muscle spasms in multiple sclerosis.

Another cannabinoid drug, called Nabilone, is sometimes used to relieve sickness in people having chemotherapy for cancer.

Trials are under way to test cannabis-based drugs for other conditions including cancer pain, the eye disease glaucoma, appetite loss in people with HIV or AIDS, and epilepsy in children.

We won't know whether these treatments are effective until the trials have finished.